Artificial socket

ABSTRACT

The present invention relates to a socket for an artificial hip joint. The present invention provides an artificial joint socket for knocking into a bone, with an excellent primary stabilization, which may be manufactured so that at least two locking elements are arranged in the distal superficies region of the socket shell. The locking elements anchor the socket shell in the hip bone and safeguard against tensile forces, torsion forces, and combined tensile and torsion forces. The locking elements effectively prevent the socket from rotating-out opposite the knock-in direction. When knocking in the implant, the locking elements cut into the bone and rotate the socket shell about the socket axis by a few degrees. The socket of the present invention forms a gradient of web-like locking elements with respect to the socket base surface, increasing from the distal or equatorial end towards the proximal or pole-side end, enabling the locking elements to jam and lock the implant against axial tensile forces, radial torsion forces, as well as the combination of both forces.

FIELD OF INVENTION

The present invention relates generally to an artificial joint socket.

BACKGROUND OF THE INVENTION

An artificial hip joint usually comprises a joint socket provided withan insert into which the head of a shank engages. Analogous to thenatural joint to be replaced, the artificial joint socket must bemounted in the socket of the pelvic bone of the patient in apositionally accurate and stable manner. Various artificial hip socketsare known, with two types of socket being most commonly used: conicallyshaped sockets and spherical sockets. The decision to use one or theother socket type is dependant on the medical condition and thepreference of the operating physician.

Both socket types comprise a metallic outer shell, an insert, and aninlay arranged in the outer shell. The inlay is usually made of ceramic,plastics, such as polyethylene (chirulene), or metal. Such an artificialjoint socket, having a shell and an insert, has been shown to be veryreliable.

One advantage of using the spherical sockets is that during pre-millingof the spherical bed, less pelvic bone needs to be removed than with aconically-shaped socket, since the acetabulum is approximately sphericaleven with greatly deformed joints. The milling process is also easierwith spherical sockets than with conical sockets since milling does notneed to be so accurate. Further, the danger of the ileum, ischium orpubis becoming damaged due to milling too deep is much lower with aspherical bed than with a conical one.

In addition to the standard design of a spherical socket whichcorresponds to a hemisphere, press-fit designs have also beensuccessfully applied. The press-fit sockets are slightly flattened atthe pole and are somewhat larger in diameter than the pre-milledhemispherical bed in the bone. The force-fit of the socket in thepre-milled bone is thus ensured. A rough surface coating is provided onshells made of metals, such as pure titanium, to permit the intergrowthof the bone cells onto the implant, ensuring an optimal secondarystabilization in the long term. One may also fill behind the press-fitsocket with spongiosa in order to achieve an improved retention on thebone. The primary stability of a press-fit socket is usually increasedby several additional spongiosa screws or other fixation means. Asdescribed in EP-B-0,601,224 and EP-A-0,943,304, the primary stability ofa screw socket is increased by way of self-cutting threads which runaround the outside of the socket base body.

In contrast to screw sockets, spherical sockets without threads, andpress-fit sockets are pressed into the pre-milled bed in a linearmovement.

Standardized socket sizes of 44 to 66,i.e. sockets with a diameter of 44to 66 mm on the socket base surface, are common and readily available onthe market.

Various devices have been designed in order to achieve an improvedprimary stability with these socket types. An artificial joint socketwith a basal flange is described in U.S. Pat. No. 4,173,797 in the year1979, in which, in the inserted condition, the socket bears on thesurface of the bone and as a result, prevents tilting of the outershell. Additional barbs in the pole region of the convex outer side ofthe shell are also described. The barbs act as rotational sacramentsbefore the implant grows in. U.S. Pat. No. 5,972,032 describes anothertype of spherical implant having barb-like projections, or spikes, inthe pole region. The press-fit socket shown in this reference is knockedin essentially along the longitudinal or rotation axis of the shellbody, and the socket is fixed in a rotationally secure manner in theknocked-in condition by way of the spikes, which are arrangedessentially parallel to the longitudinal or rotation axis of the implantin the pole region. These spikes, however, only slightly contribute tosafeguarding against tensile loads. It is known that compressive forceswith a significant axial component in the femur may be built up withpress-fit sockets. Moreover, the above described spikes may notprimarily safeguard against these forces. Two additional solutions tothe problem of insufficient primary securement are described in U.S.Pat. No. 5,755,799 and U.S. Pat. No. 6,231,612. In order to permit ascrewless and cementless fixation in the bed of the bone, both implantsin the equatorial region of the outer side of the shell comprise amultitude of small projections. The tips of these scale-like ortile-like barbs point distally with respect to the body of the patient,and are thus counter to the knock-in direction. On account ofmanufacturing technology, the tips follow the contour of the superficiesexactly, and the individual rows of scales run from the shell opening inthe direction of the pole parallel to the knock-in direction. Themanufacture of such implants is very complicated and is thus veryexpensive. In addition, the barbs are only a few tenths of a millimeterhigh, and often do not provide the freshly inserted implant with therequired amount of primary stability.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide anartificial joint socket which does not have the above-mentioneddisadvantages and ensures a particularly simple and exact implantationand a reliable primary fixation. Another object of the invention is toprovide optimal recovery of the anatomical function of the socket of thehip joint with a physiological introduction of force.

The present invention provides an artificial joint socket for knockinginto a pre-milled bed in the bone, with an excellent primarystabilization, in which at least two locking elements are arranged inthe distal superficies region of the socket shell. The inventive lockingelements anchor the socket shell in the hip bone and safeguard againsttensile forces, torsion forces, and combined tensile and torsion forces.The inventive locking elements at first appear similar to known websformed of a coarse thread. However the geometry of the inventive lockingelements deviates from that of common thread webs so that theyeffectively prevent the socket from rotating-out opposite the knock-indirection. On knocking in the implant, the locking elements cut into thebone and rotate the socket shell about the socket axis by a few degrees.In a preferred embodiment, the gradient of the web-like locking elementswith respect to the socket base surface, increases from the distal, orequatorial, end towards the proximal, or pole-side end, thereby enablingthe locking elements to jam and lock the implant against axial tensileforces, radial torsion forces, as well as combinations of both forces.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and embodiments of the present invention areexplained in the following description with reference to the attacheddrawings, wherein:

FIG. 1 a shows a cross-sectional view of a press-fit socket known in theart, wherein only a shell is shown and the inlay is omitted;

FIG. 1 b shows a top view of the socket of FIG. 1 a, illustrating theinside of the shell;

FIG. 2 shows a partial cross-sectional view of the press-fit socketaccording to the present invention, in the region of a locking element,wherein only the shell is shown and the inlay has been omitted;

FIG. 3 shows a partial view of a locking element of another embodimentof the joint socket of the present invention, viewed from the proximalend or from the pole;

FIG. 4 shows a schematic lateral view of a linear locking element;

FIG. 5 a shows a side view of a distal superficies region of a sphericalsocket with a locking element as shown in FIG. 3;

FIG. 5 b shows a side view of a distal superficies region of a sphericalsocket with another embodiment of the locking element; and

FIG. 5 c shows a side view of the distal superficies region of aspherical socket with a further embodiment of the locking element.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a shows a cross-sectional view of a base body or shell 10 of apress-fit socket according to known in the art. This type of press-fitsocket deviates from the hemispherical shape by its increased flatteningin the direction of the pole region 12. The base body 10, as indicatedby the dashed lines, may be divided into four socket layers. Theassociated superficies in each layer have a radius R₁, R₂, R₃, R₄ whichincreases from the distal end 14, i.e. distant to the pole, to theproximal end. The shown sectional plane contains the axis of the socket.The inside of the shell 10 comprises a recess 16 for accommodating aceramic or plastic inlay. This inlay, not shown in the figure, serves asa counter bearing for the ball head of a femoral prosthesis, alsoreferred to as a shank.

The socket according to the present invention has all the essentialadvantages of the known press-fit socket shown in FIG. 1 a, with regardto material selection, surface coatings and socket geometry. One elementof the known socket which has been omitted from the socket of thepresent invention is the bores B₁, B₂ and B₃, shown in FIG. 1 b. Thebores B₁, B₂, B₃ accommodate spongiosa screws, which provide additionalprimary stabilization of the implant in the bone.

FIG. 2 shows a half cross-sectional view of a socket 1, designed for anartificial hip joint. A socket axis A_(p) lies in the section plane. Abase body or a shell 10 is provided with a spherical or ellipsoidalsuperficies, or outer surface 11, which is essentially rotationallysymmetrical to the socket axis A_(p). At least two locking elements 20are provided on the outer side of the base body 10. In FIG. 2, only oneof the at least two locking elements 20 is shown. The locking element 20comprises a knock-in web 21 which in the embodiment of FIG. 2, istoothed.

The locking elements 20 are distributed symmetrically over the peripheryof the socket 1 so that the resulting forces from the socket 1 beingknocked-in are uniformly distributed and thus the socket does not tilt.In a preferred embodiment, the locking elements 20 are arrangeduniformly distanced to one another. If three locking elements 20 areprovided, the elements 20 are arranged in the positions at 0°, 120°, and240° with respect to a circular socket base surface G_(P). If fourelements 20 are provided, the elements 20 are arranged in the positionsat 0°, 90°, 180°, and 270° with respect to a circular socket basesurface G_(P). The elements 20 may also be distanced irregularly fromone another as long as the symmetry is retained, such that four elementsmay be arranged in the following positions: 0°, 60°, and 180°, and 240°.

In FIG. 3, a partial view of an embodiment of a locking element 20′ of ajoint socket according to the present invention is shown in a view fromthe pole. The shown locking element 20′ comprises a knock-in web 21which has a particularly preferred geometry. A distal, or equatorial,end of the knock-in web 21 is further away the socket base surface G_(p)while a proximal, or pole-side, end of the knock-in web 21 is closer tothe socket base surface G_(p). Knock-in web 21 is angled with respect tosocket axis A_(p). The gradient, with respect to socket axis A_(p),increases continuously from the distal, or equatorial, end to theproximal, or pole-side, end of the web 21. When the inventive jointsocket is advanced into a prepared cavity, the angular deviation ofknock-in web 21 with respect to socket axis A_(p) allows the jointsocket to twist about a polar axis. The angular deviation, or gradient,of knock-in web 21 is defined by the angle measured between socket basesurface G_(p) and knock-in web 21. FIG. 4 shows a lateral view of ajoint socket showing a more simply designed web 21 which assumes alinear course with a constant gradient. The contour of the web accordingto FIG. 3 is shown dashed in FIG. 4 in order to emphasize the increasinggradient of the knock-in web 21 of FIG. 3. An angle of twist is definedby an angle measured between socket axis A_(p) and knock-in web 21. Inthe embodiments shown in FIGS. 3 and 4, both webs have a gradient of 75°with respect to the socket base surface G_(p), which corresponds to atwist angle of 15°. In other embodiments, the gradient of the knock-inwebs 21 from the distal end of the web to the proximal end of the web 21may be between 85° to 60°, preferably 80° to 70°, and most preferably75° with respect to the socket base surface G_(p). The twist angle isaccordingly between 5° to 30°, preferably 10° to 20°, and mostpreferably 15°.

The curvature, or the increasing gradient, of the web according to FIG.3 with respect to the base surface leads to the fact that the socketshell, after knocking into the hipbone, is jammed and locked so that arotating-out counter to the knock-in direction is effectively prevented.The locking elements 20 thus effectively safeguard the implant againsti) axial tension forces, ii) radial torsion forces, as well as iii) acombination of both types of forces.

FIG. 3 additionally shows that the width of the web 21 preferablyreduces from the web base on the superficies 11 over the whole height ofthe web. The preferred embodiment of the web 21 shown in FIG. 3 istrapezoidal in cross section and is provided with converging web flanks22, 23, which enclose an angle γ of 15° to 21°, preferably 18°. The webflanks may also be formed parallel to one another, however the knock-inhas been shown to be simpler with the illustrated trapezoidal webs.

As shown in FIG. 3, independent of the size of the joint sockets, theheight of the web H_(S), H_(S′) is between 0.5 and 4 mm, preferablybetween 1.8 and 2.6 mm. These magnitudes have been shown to beadvantageous in view of the physiological properties of the bone intowhich the webs are knocked.

As shown in FIG. 3, the width B_(S) as well as the height H_(S) of theweb 21 reduces from the distal, or equatorial, end to the proximal, orpole-side, end. A lateral view of the web of FIG. 3 is shown in FIG. 5a. As shown in FIG. 5 a, a cutting surface 24 of the knock-in web 21follows a circular arc with a radius R_(p) which is smaller than thesuperficies radius R_(S) of the corresponding ball layer so that theextension of the cutting surface 24 converges with the superficies 11.The web height H_(S) at the distal or equatorial end is between 1 and 4mm, preferably 2.4 and 2.8 mm. At the proximal end, the web height H_(S)is between 0.5 and 3 mm, preferably 1.5 to 1.8 mm. The height thuscontinuously reduces from the distal to the proximal or pole-side end ofthe web 21.

As shown in FIG. 5 b, the web 21 proceeding from the distal socket basesurface G_(P) extends only roughly up to half the socket height in thedirection of the pole. If the socket were lopped at the proximal web endthen a distal spherical layer of the base body 10 would remain, whoseheight H_(K) would then correspond to between 20 to 30%, preferably 24to 26% of the socket diameter D_(P). Since the socket needs to beknocked into a pre-milled spherical bed in the bone, the cutting andjamming effect of the knock-in webs 21 reduces with an increasingdistance to the socket base surface G_(P). Web portions in the poleregion would no longer contribute significantly to the securing of theimplant according to the invention, but would only be further axiallysqueezed into the bone, similar to the known spikes.

The configuration of a socket for conical implants is different fromthat of a spherical socket. For a conically-shaped socket, the lockingelements may advantageously extend over approximately the entire heightof the socket.

In the embodiments shown in FIGS. 2, 5 b and 5 c, the joint socket isprovided with locking elements 20 that do not comprise continuous webs21, but rather, comprise interrupted webs 21 wherein cutting andclamping teeth 210, 211, 212, 213 are formed. The teeth simplify theknock-in of the implants and provide additional surface area for a laterintergrowth of bone cells on the webs. A further advantage of the teethis that the distal rear sides of the teeth lock the implant particularlyeffectively in a bone upon tensile loading.

An improved cutting effect on knocking in is achieved in that the uppercutting surfaces 240, 241, 242 of the proximal teeth 210, 211, 212 havea clearance. The cutting surfaces 240, 241, 242 are thus slanted about aclearance angle β with respect to the cutting circular arc with theradius R_(S), so that an actual cutting edge 251 arises at therespective end face 250 of a tooth 211.

As shown in FIGS. 3 and 5 c, in addition to the clearance of theproximal teeth 210, 211, 212, a further measure is provided forsimplifying the knocking in. The respective first proximal cutting orjamming tooth 210 of a locking element 20′ comprises a front cuttingsurface 214 which is positioned at an effective cutting angle a withrespect to the socket base surface G_(P). The front cutting surface 214thus does not lie parallel to the socket base surface G_(P), but ratheris set steeper and preferably defines a plane which is approximatelyperpendicular to the superficies 11.

In the previously described embodiments of the present invention, thelocking elements 20 or 20′ were formed as one piece with the base bodyor with the shell 10. In a further embodiment which is not shown in thefigures, the locking elements 20 may be detachably fastened to the shell10. In addition, the knock-in webs 21 described above are positioned ona base or carrier element, which may be introduced into a correspondinggroove in the socket shell 10 in an exact fit manner. If the lockingelements 20 are manufactured separately from the base body 10, one mayutilize various material combinations for the shell and the lockingelements. The locking elements 20 are preferably applied and secured inthe base body 10 before knocking into the corresponding grooves in thebase body 10. It is however possible to first knock in the socket beforesecuring it with the separate locking elements 20.

1. A socket, comprising: (a) a base body having a superficies, thesuperficies being essentially rotationally symmetrical relative to avertical axis of the socket; and (b) at least two locking elementspositioned on an outer surface of the base body, wherein each of thelocking elements comprises at least one knock-in web, the knock-in webhaving a gradient from a distal web end to a proximal web end of atleast 85° to 60° with respect to a base surface of the socket lyingperpendicular to the vertical axis of the socket.
 2. The socketaccording to claim 1, wherein the shape of the superficies is selectedfrom the group consisting of: spherical, ellipsoidal, and conical. 3.The socket according to claim 1, wherein the at least two lockingelements are distributed symmetrically over the outer surface of thesocket.
 4. The socket according to claim 1, wherein the at least twolocking elements are arranged uniformly distanced to one another.
 5. Thesocket according to claim 1, wherein the gradient increases from adistal end towards a proximal end of the web, with respect to the socketbase surface lying perpendicular to the socket axis.
 6. The socketaccording to claim 1, wherein a cutting radius of each of the at leasttwo locking elements is smaller than a radius of the superficies of thebase body, whereby an extension of a cutting surface converges with thesuperficies.
 7. The socket according to claim 1, wherein the at leastone web extends over a distal region of the superficies of the basebody, and wherein the height of the region correspond to between 20 to30% of the socket diameter.
 8. The socket according to claim 1, whereinthe height of each of knock-in web is between 0.5 and 4 mm.
 9. Thesocket according to claim 8, wherein the height of the knock-in webreduces continuously from a distal end towards a proximal end of theweb, and wherein the height of the web at the distal end is between 1and 4 mm, and the height of the web at the proximal end is between 0.5and 3 mm.
 10. The socket according to claim 1, wherein the width of theat least one web in its cross section reduces from the base of the webon the superficies over the entire height of the web.
 11. The socketaccording to claim 10, wherein the at least one web is trapezoidal incross section and comprises at least two web flanks, which converge toone another and enclose an angle between 15° to 21°.
 12. The socketaccording to claim 1, wherein the width and the height of each of theknock-in web reduces from a distal end towards a proximal end.
 13. Thesocket according to claim 1, wherein the at least one web comprises atleast two cutting and jamming teeth.
 14. The socket according to claim13, wherein the at least two teeth each comprise an upper cuttingsurface which is arranged at least partly at a clearance angle withrespect to the radius of a cutting circular arc.
 15. The socketaccording to claim 13, wherein a proximal cutting and jamming toothfurther comprises a front cutting surface which is arranged at aneffective cutting angle with respect to the base surface of the socket.16. The socket according to claim 1, wherein the at least two lockingelements are integrally formed with the base body.
 17. The socketaccording to claim 1, wherein the at least two locking elements may bedetachably fastenable to the base body.
 18. The socket according toclaim 1, wherein the at least one knock-in web defines a gradient of 80°to 70° from a distal web end to a proximal web end with respect to thebase surface, corresponding to an angle of twist of 10° to 20°.
 19. Thesocket according to claim 1, wherein the knock-in web further defines anangle of twist with respect to the axis of socket of at least 5° to 30°.20. The socket according to claim 1, wherein the socket is selected froma group consisting of: joint socket for an artificial hip joint,press-fit socket for an artificial hip joint, and joint prosthesis forknocking into a bone.